1. Field of the Invention
The present invention relates generally to methods and devices for the nondestructive evaluation of materials. The present invention relates more specifically to the use of magnetostrictive sensors to inspect electric resistance welds.
2. Description of the Related Art
Magnetostrictive effect refers to the phenomena of a physical dimension change in ferromagnetic materials that occurs through variations in magnetization. In magnetostrictive applications, the generation and detection of mechanical waves is typically achieved by introducing a pulse current into a transmitting coil adjacent to a ferromagnetic material. The change in magnetization within the material located near the transmitting coil causes the material to change its length locally in a direction parallel to the applied field. This abrupt local dimension change, which is the magnetostrictive effect, generates a mechanical wave that travels at the speed of sound within the ferromagnetic material. When the mechanical wave is reflected back from the end of the ferromagnetic material, or from a defect in the ferromagnetic material, and reaches a detection coil, the mechanical wave generates a changing magnetic flux in the detection coil as a result of the inverse magnetostrictive effect. This changing magnetic flux induces an electric voltage within the detection coil that is proportional to the magnitude of the mechanical wave. The transmitting coil and the detection coil can be identical.
Advantages of using the magnetostrictive effect in nondestructive evaluation (NDE) applications include (a) the sensitivity of the magnetostrictive sensors, (b) durability of the magnetostrictive sensors, (c) no need to couple the sensor to the material being investigated, (d) long range of the mechanical waves in the material under investigation, (e) ease of implementation, and (f) low cost of implementation.
The use of magnetostrictive sensors (MsS) in the nondestructive evaluation (NDE) of materials has proven to be very effective in characterizing defects, inclusions, and corrosion within various types of ferromagnetic and non-ferromagnetic structures. A MsS launches a short duration (or a pulse) of elastic guided waves in the structure under investigation and detects guided wave signals reflected from anomalies such as defects in the structure. Since guided waves can propagate long distances (typically 100 feet or more), the MsS technique can inspect a global area of a structure very quickly. In comparison, other conventional NDE techniques such as ultrasonics and eddy current inspect only the local area immediately adjacent to the probes used. Therefore, the use of magnetostrictive sensors offers a very cost effective means for inspecting large areas of steel structures such as strands, cables, pipes, and tubes quickly with minimum support requirements such as surface preparation, scaffolding, and insulation removal. The ability to use magnetostrictive sensors with little preparation of the object under inspection derives from the fact that direct physical contact between the sensors and the material is not required.
Efforts have been made in the past to utilize magnetostrictive sensor technologies in association with the inspection of both ferromagnetic and non-ferromagnetic materials. Included in these efforts are systems described in U.S. Pat. Nos. 5,456,113; 5,457,994; and 5,501,037, which are each commonly owned by the assignee of the present invention. The disclosures of U.S. Pat. Nos. 5,456,113; 5,457,994; and 5,501,037, provide background on the magnetostrictive effect and its use in NDE and are therefore incorporated herein by reference. These efforts in the past have focused primarily on the inspection of pipe, tubing, and steel strands/cables wherein the geometry of the structure is such that the cross-sectional diameter is small in comparison to the length of the structure. While these systems and their application to longitudinal structures find significant applications, there are yet other structures that could benefit from the use of magnetostrictive based NDE.
Other efforts have been made in the past to utilize sensors that measure magnetic flux and/or acoustic waves in structural materials. These efforts have included those described in the following patents:
U.S. Pat. No. 3,555,887 issued to Wood on Jan. 19, 1971 entitled Apparatus for Electroacoustically Inspecting Tubular Members for Anomalies Using the Magnetostrictive Effect and for Measuring Wall Thickness. This patent describes a system designed to direct a mechanical wave through the thickness dimension of a long tubular member. The sensitivity of the device is limited to the directing of a wavefront normal to the surface of the material under inspection and immediately back to a sensor when reflected from an opposite wall or an anomaly.
U.S. Pat. No. 4,881,031 issued to Pfisterer, et al. on Nov. 14, 1989 entitled Eddy Current Method and Apparatus for Determining Structure Defects in a Metal Object Without Removing Surface Films or Coatings. This patent describes a method for establishing localized eddy currents within ferromagnetic materials and recognizes the presence and effect of a coating in order to identify and quantify corrosion beneath the coating. As with other eddy current methods, the ability to inspect a material is limited to the area immediately adjacent to the sensor.
U.S. Pat. No. 5,544,207 issued to Ara, et al. on Aug. 6, 1996 entitled Apparatus for Measuring the Thickness of the Overlay Clad in a Pressure Vessel of a Nuclear Reactor. This patent describes a system directed solely to the measurement of magnetic field variations that result from the distribution of the magnetic field through overlays of varying thickness. The system utilizes a magnetic yoke that is placed in close contact with the surface of the overlay clad of the pressure vessel.
U.S. Pat. No. 5,687,204 issued to Ara, et al. on Nov. 11, 1997 entitled Method of and Apparatus for Checking the Degradation of a Pressure Vessel of a Nuclear Reactor. This patent describes a system similar to the earlier issued Ara, et al. patent and utilizes a magnetic yoke having an excitation coil and a magnetic flux measuring coil that are placed in close contact with the inner wall of the pressure vessel. The hysteresis magnetization characteristics formed by the magnetic yoke and the pressure vessel wall are measured. Degradation of the material comprising the pressure vessel is inferred from a determination of the hardness of the material which is determined from the coercive forces obtained by analyzing the hysteresis characteristics of the magnetization.
The nondestructive evaluation of materials using magnetostrictive sensors is based upon the magnetostrictive effect and its inverse effect, and the phenomenon that causes the physical dimensions of a ferromagnetic material to change slightly when the material is magnetized or demagnetized or otherwise experiences a changing magnetic field. The inverse effect is a phenomenon that causes a magnetic flux in the material to change when the material is stressed. Systems utilizing magnetostrictive sensors use the magnetostrictive effect and its inverse effect to generate and detect guided waves that travel through the ferromagnetic material.
In general, a magnetostrictive sensor consists of a conductive coil and a means for providing a DC bias magnetic field in the structure under inspection. The means for providing a bias magnetic field can include the use of either permanent magnets or electromagnets. In a transmitting magnetostrictive sensor, an AC electric current pulse is applied to the coil. The resulting AC magnetic field (a changing magnetic field) produces elastic waves (also known as guided waves) in an adjacent ferromagnetic material through the magnetostrictive effect. For pipes, cables, tubes, and the like, the waves are launched along the length of the longitudinal structure. In the receiving magnetostrictive sensor, a responsive electric voltage signal is produced in the conductive coil when the elastic waves (transmitted or reflected from anomalies within the material) pass the sensor location, through the inverse magnetostrictive effect.
With MsS techniques, defects are typically detected by using the pulse-echo method well known in the field of ultrasonics. Since the sensor relies on the magnetostrictive behavior found in ferromagnetic materials, this technology is primarily applicable to the inspection of ferromagnetic components such as carbon steel piping or steel strands. It is also applicable, however, to the inspection of nonferrous components if a thin layer of ferromagnetic material, such as nickel, is plated or coupled onto the component in the area adjacent to the magnetostrictive sensors.
The magnetostrictive sensor technique has the advantage of being able to inspect a large area of material from a single sensor location. Such sensors have, for example, been used to accurately inspect a length of pipe or cable of significantly more than 100 feet. Further, magnetostrictive sensor techniques are comprehensive in their inspection in that the methods can detect both internal and external defects, thereby providing a 100% volumetric inspection. The techniques are also quite sensitive, being capable of detecting a defect with a cross-section less than 1% of the total metallic cross-section of cylindrical structures such as pipes, tubes, or rods. Finally, as indicated above, magnetostrictive sensor techniques do not require direct physical contact between the component surface and the sensor itself. This eliminates the need for surface preparation or the use of a couplant.
In recent years, there have been many reported occurrences of steel liner containment vessels degrading at commercial nuclear power plants. Due to the aging of such facilities and the increased requirements for inspection, incidents of degradation are likely to increase. The structural degradation of these liners, especially corrosion damage, is an important concern since the liners are designed to provide a leak-tight pressure boundary for the nuclear material. Typically, the containment vessels are made from steel sheets welded together. The welds are part of the vessel that should also be inspected.
Many other industrial uses of plate type ferromagnetic materials could benefit from more frequent inspections to determine the state of deterioration, the location of faults, and the likelihood of failure. In most instances in the past, inspections of large plate type objects have required either very expensive off-line inspections or statistical samplings of randomly selected local areas that are for the most part less than reliable. It has heretofore been difficult to carry out a thorough inspection of a plate type structure, or a structure comprised of a plurality of plate type sheets of material welded together, without high cost and long down time for the object under inspection. It would be desirable to use the magnetostrictive sensor technique for detecting and locating various anomaly characteristics within plate type materials. Such techniques could be used for detecting and locating wall thickness reductions in liners, such as those described above, that might be caused by corrosion over time. If such a system were applicable, it would be possible to inspect welds or otherwise inaccessible regions of containment liners and the like that are either imbedded in concrete or adjacent to flooring or equipment that cannot be moved.
It would therefore be desirable to implement magnetostrictive sensor techniques in conjunction with plate type structures in a manner similar to, and with the accuracy of, such systems utilized in conjunction with cylindrical structures to inspect welds in such plate type structures. It would be desirable if an inspection of welds in plate type structures could be carried out in an efficient manner that did not require full access to the surface of the plate. Such a magnetostrictive sensor system would be able to investigate welds in large plate type structures or pipes and would provide a cost effective global inspection of welds in the structure.
In steel mills, coils of relatively thin sheet steel are wound on a drum. Individual coils are joined together by flash butt welding the tail end of the leading coil to the head of a subsequent coil. Flash butt welding involves a rapid heating of coil ends and upset forging of the heated ends. Excess material from the top and bottom of the weld line is trimmed. During the flash butt welding, the joined coils are held stationary for flash butt welding and trimming. At a subsequent inspection station, the flash butt welding is momentarily stopped for non-destructive inspection. If the flash butt welding is good, the joined coils are released for subsequent processing. If a defect is found in the flash butt welding, the coil is moved back to the welding station and the coils are rewelded after removing the bad weld. The entire welding, trimming, and inspection are performed in less than one minute. This is to ensure uninterrupted flow of the downstream operations which can typically reach a flow speed of 30 miles per hour.
During downstream processing, the coil steel is subject to co-rolling, pickling, and annealing, during which time the flash butt welds are subjected to thickness reduction and high tensile stresses. Under these high tensile stresses, a properly made butt weld will often fail causing coil breakage, which interrupts the operation completely. There is a need to have some type of reliable way to inspect the welds to reduce and eliminate coil breakage during downstream operation.
The current method used to inspect the welds is electromagnetic acoustic transducers (EMATs). However, EMAT inspection is not good on sheets more than 2.5 millimeters thick. Also, because the EMAT inspection range is short, the weld joint must be moved out of the welding and trimming machine to a location where the EMAT inspection is performed. If the type of inspection had a longer range, the coil could be inspected while it is still in the welding and trimming machine, thus shortening the process time and increasing productivity.
It is therefore an object of the present invention to provide a method for implementing magnetostrictive based NDE of welds in association with plate type structures and to determine the presence of anomalies indicative of defective welds.
It is a further object of the present invention to provide a method for using magnetostrictive sensors for the inspection of welds in plate type structures that is capable of transmitting and receiving guided waves within the plate type structures and generating signals representative of the characteristics of such waves appropriate for the analysis and detection of anomalies in the welds.
It is a further object of the present invention to provide a method for the inspection of plate type structures and welds therein that includes the use of a magnetostrictive sensor specifically adapted for directing guided waves into the plate type structures and detecting such waves as may be reflected from anomalies within the structure including welding defects.
It is a further object of the present invention to provide a method and apparatus for the nondestructive evaluation of plate type structures and welds therein utilizing magnetostrictive sensors that are capable of investigating large volumes of welded plate type structures without access to the entire surface area of the plates or the welds.
It is yet another object of the present invention to provide a method and apparatus for nondestructive evaluation of welded plate type structures or containments having ferromagnetic materials through the use of a magnetostrictive sensor that may operate either in the symmetrical or anti-symmetrical Lamb wave mode.
It is yet another object of the present invention to provide a method and apparatus for nondestructive evaluation of welded plate type structures utilizing magnetostrictive sensors that generate and detect shear horizontal waves in the item being inspected.
It is yet another object of the present invention to provide a method and apparatus for inspecting electric resistance welding utilizing magnetostrictive sensors.
It is still another object of the present invention to provide a method and apparatus for nondestructive evaluation of pipes and welds therein using magnetostrictive sensors that propagate guided waves in a circumferential direction around the pipe.
Yet another object of the present invention is to provide a method and apparatus for inspecting butt welds in sheet steel using magnetostrictive sensors that propagate guided waves along the length of the sheet.
In fulfillment of these and other objectives, the present invention provides a method and apparatus for implementing magnetostrictive sensor techniques for the nondestructive evaluation of plate type structures such as walls, vessels, enclosures, and the like and welds therein. The system includes magnetostrictive sensors specifically designed for application in conjunction with welded plate type structures that generate guided waves in the plates which travel through the plate in a direction parallel to the surface of the plate. Similarly structured sensors are positioned to detect the guided waves (both incident and reflected) and generate signals representative of the characteristics of the guided waves detected. The system anticipates the use of either discrete magnetostrictive transmitters and receivers or the use of a single magnetostrictive sensor that operates to both transmit and detect the guided waves. The sensor structure is longitudinal in nature and generates a guided wave having a wavefront parallel to the longitudinal direction of the sensor. Appropriate electronics associated with the process of generating the guided waves and controlling the propagation direction of the generated wave through the magnetostrictive transmitter as well as detecting, filtering, and amplifying the guided waves at the magnetostrictive receiver, are implemented as is well known in the art. Signal analysis techniques, also known in the art, are utilized to identify anomalies within the plate type structure and welds therein. The method utilizes pattern recognition techniques as well as comparisons between signal signatures gathered over time from the installation of the structure under investigation to a later point after deterioration and degradation may have occurred.
By rotation of the magnetic field by 90xc2x0, the magnetostrictive sensor can be changed from operating in the symmetrical or the anti-symmetrical Lamb wave mode to a horizontal shear wave that is applied to the ferromagnetic material being inspected. In the horizontal shear wave mode, the DC bias magnetic field is in a direction perpendicular to the direction of wave propagation.
The magnetostrictive sensors can also be used to detect defects in electric resistance welding, such as pipes that are welded along a scam thereof. For example, a magnetostrictive transmitter can be placed on one side of the pipe being investigated and a magnetostrictive receiver on the other side of the pipe. By generation of a guided wave around the pipe, any defects in the pipe can immediately be detected, such as in the area of the weld.
Also, the magnetostrictive sensors can be used to detect defects in welding joints between coils of sheet steel. The inspection can occur while the weld joint is still within the welding and trimming machine so that if additional welding and trimming is necessary, it can be performed before the weld joint moves down the processing line. The elastic wave will move in a direction perpendicular to the weld joint so that defects in the weld joint can be more easily determined.